Henry Cavendish calculated Earth's average density in the Cavendish experiment.
Published in 1798, the experiment used a torsion balance designed by geologist John Michell and built for Cavendish. Small lead spheres were attracted gravitationally to larger lead spheres, causing a delicate suspended rod to twist.
By measuring the tiny gravitational force between the laboratory masses and comparing it with Earth's pull, Cavendish estimated Earth's mean density at about 5.48 times that of water. The modern value is approximately 5.51 times the density of water.
The experiment is often called a measurement of the gravitational constant, but Cavendish himself framed his result as finding Earth's density. His work provided the first reliable laboratory measurement of gravitational attraction between known masses and helped determine Earth's mass indirectly.